xen/arm: clear UIE on hypervisor entry

xen/arm: clear UIE on hypervisor entry

UIE being set can cause maintenance interrupts to occur when Xen writes
to one or more LR registers. The effect is a busy loop around the
interrupt handler in Xen
(http://marc.info/?l=xen-devel&m=141597517132682): everything gets stuck.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reported-and-Tested-by: Andrii Tseglytskyi <andrii.tseglytskyi@globallogic.com>
Tested-by: Julien Grall <julien.grall@linaro.org>
Release-acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>